Add leasing backup to distributed garbage collection

When a client hard fails or disconnects, none of the referenced objects on the client side can possibly be garbage collected on the server side.

The pinging mechanism keeps the connection alive (if so desired) and can be leveraged to refresh leases on the server side

References should be dropped on the server side for sessions that have not 'recently' been resfreshed.

A new thread will need to be created to look for expired sessions.